Solid oxide fuel cell (SOFC) technology is being developed for automotive and stationary applications. It is known that mixed ionic and electronic conducting (MIEC) perovskite-type ABO3 oxides are promising cathode materials for solid oxide fuel cells and oxygen semi-permeable membranes.
The general chemical formula for perovskite compounds is ABX3, wherein ‘A’ and ‘B’ are two cations of very different sizes, and X is an anion that bonds to both. (The native titanium mineral perovskite itself is of the formula CaTiO3). The ‘A’ atoms are larger than the ‘B’ atoms. The ideal cubic-symmetry structure has the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations, or both are reduced.
In the LSCF pervoskite crystal lattice, the A-sites are occupied by La and Sr ions, and the B-sites are occupied by Co and Fe ions that surround oxygen ions. In these materials, the cathode oxygen exchange reaction in an SOFC is not limited only to the triple-phase boundary line between electrolyte, cathode, and gas phase, but extends over a large three-dimensional area within the cathode.
Compositions of the general formula La1−xSrxCo1−yFeyO3−δ have been proposed in the prior art as materials for SOFC cathodes due to their high catalytic activity for the oxygen exchange reaction and a high electronic conductivity for current collection. The physical and chemical properties of this class of materials, such as electrical conductivity, electronic structure, catalytic activity, stability, and thermal expansion coefficient (TEC), have been studied in detail. Generally, electronic and ionic conductivities and catalytic activity are enhanced with increasing values of x and decreasing values of y, whereas there is an opposite tendency for chemical stability.
Further, it is known that these properties are strongly affected by a change in the combination of La and Co oxide concentrations. Electronic conductivity for La0.6Sr0.4Co0.2Fe0.8O3−δ (also known in the art as “LSCF 6428”) is sufficient (>250 S/cm at 1073° K) for the above-mentioned applications. However, ionic conductivity is rather low (˜10−2 S/cm at 1273° K). It has been suggested that increasing Sr-deficiency in LSCF 6428 is a way to improve its oxide ionic conductivity and catalytic property for oxygen reduction.
Another investigation on A-site deficiency has reported significant decrease in electronic conductivity in the order La0.6Sr0.4−z>(La0.6Sr0.4)1−z>La0.6−zSr0.4. In yet an another investigation, it has been concluded that the TEC decreases with decreasing A-site stoichiometry, and that electronic conductivity of the perovskites has a weak dependence on the A-site stoichiometry. As a result, (La0.6Sr0.4)1−z Co0.2Fe0.8O3−δ materials are commonly used for SOFC cathodes and are available commercially.
From the above discussion it is clear that while A-site deficiency is desirable, both ionic and electronic conductivities need to be high to drive performance in terms of fuel cell power density (W/cm2) and to reduce the cost. This is because cathode polarization, or resistance associated with the low rate of chemical and electrochemical reactions occurring in the cathode, is still the major source of voltage loss in SOFCs. Control of cathode microstructure (pore size, shape, and porosity) helps, but supply of electrons (electronic conductivity) and oxygen (oxide ion conductivity) deep into the cathode is the key to reducing cathode losses.
The supply of oxygen ions to the electrolyte is a function of ionic conductivity on one hand and the supply of electrons on the other hand and depends on the electronic resistivity of the cathode material. What is desired is a cathode material that has high electronic and ionic conductivities along with high catalytic activity. Further, cathode polarization resistance, mechanical properties, and cost are major concerns in the development of a practical SOFC.
Still further, reducing cost of manufacture requires a reduction in fuel cell operating temperature to around 750° C. or below so that less expensive interconnect and sealing materials can be used. The degradation of these materials is reduced at lower operating temperature, and thus reliability and long-term stability of a fuel cell stack is improved. Development of such materials can help in attaining higher power density (W/cm2) at lower cost from SOFC stacks.
What is needed in the art is improved LSCF perovskite materials having enhanced ionic and electronic conductivity, improved thermal properties, and capability for yielding higher SOFC power densities and at lower operating temperatures.
It is a principal object of the present invention to reduce the cost of manufacture and improve performance and stability of a solid oxide fuel cell.